DESCRIPTION: provided by applicant): Protein synthesis is a highly conserved process that is essential and common to all forms of life. The majority of mRNAs include an upstream, untranslated leader sequence that contains translational signals for binding ribosomes and establishing the start site for translation. Translation initiation is believed to involve a binding of the mRNA to the small ribosomal subunit (30S), followed by binding of the large subunit (50S), and assembly of the 70S ribosome. The experiments described in this proposal develop our recent observations that leaderless mRNAs bind in a tRNA-dependent manner to the small or the large ribosomal subunit, suggesting the potential for a translation initiation mechanism that differs dramatically from translation of all other messages. Several lines of evidence support the notion that leaderless mRNAs might initiate translation in vivo through interaction with the 50S subunit. These results have significant implications for novel translational regulation and ribosome function. A detailed analysis of interactions between 50S subunits and leaderless mRNA might allow for the design and development of novel antimicrobial compounds that specifically target the 50S subunit. Leaderless mRNAs provide an excellent research tool to probe mRNA:rRNA and mRNA:ribosome interactions, as well as functional properties of ribosomes and ribosomal subunits that are not available with conventionally leadered mRNAs. Also, the widespread occurrence of leaderless mRNAs suggest that our results with Escherichia coli will likely be true for leaderless mRNAs in other translation systems as well.